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Nonlinear optical switch based on two-dimensional material is the core device of Q-switched fiber laser. The concentration of two-dimensional material optical switch will directly affect its nonlinear optical absorption characteristics, thus changing the time-domain characteristics of the pulse. Therefore, the influence of two-dimensional material concentration on Q-switched optical pulse is studied, and a saturable absorption optical switch based on Ti: Bi2Te3 with different concentration is made through experiments. The influence of Ti: Bi2Te3 concentration on nonlinear optical absorption characteristics is analyzed, and the relationship between the modulation depth, pulse width, repetition frequency and single pulse energy of Q-switched optical pulse with different concentration Bi2Te3 is obtained. Finally, the Ti: Bi2Te3 concentration is optimized for the resonator parameters. When the pump power is 71 MW, the central wavelength is 1560 nm, the pulse width is 8 μ s, the repetition frequency is 14.2 kHz, the average output power is 2.15 MW, and the corresponding single pulse energy is 151.4 NJ.

Gadoped ZnO(GZO) thin films were grown on single crystal silicon(111) and quartz glass substrates by pulsed laser deposition(PLD) method at different substrate temperatures from room temperature to 750 ℃.The change of substrate temperature leads to different diffusion and desorption rates for adsorption atoms resting on substrate surface,and results in different crystalline quality for GZO thin films.The GZO thin film grown at 450 ℃ has the best crystalline quality.The carrier concentration monotonically decreases with the increase of substrate temperature,which is related to the intrinsic defects in films.The Hall mobility first increases and then decreases with the substrate temperature increasing,it results from the variation of grain boundary scattering.And the lowest resistivity ~0.02 Ω·cm is obtained in the film prepared at 450 ℃.With the increase of substrate temperature,the optical band gap becomes narrow due to the monotonic increase of carrier concentration.The average transmittance in visible region is above 85% for all the GZO samples.Our results exhibit that the optical and electrical properties of GZO thin films prepared by PLD method can be adjusted by the change of substrate temperature.

The effects of interband transition,free carrier absorption and bandgap narrowing on linewidth enhancement factor(α factor) in semiconductor lasers were comprehensively considered in a simple model.A convenient calculation method of α factor in semiconductor lasers was presented.The formula for α factor was derived at first,the gain of GaAs semiconductor lasers was theoretically analyzed and calculated,and the process of solving the Fermi integral function by taking advantage of the Mupad notebook in MATLAB software was introduced.Further,the effect of interband transition on α factor was calculated based on the peak variation of gain fitting curves.Finally,both the effects of free carrier absorption and bandgap narrowing on α factor were disscused,respectively,and their values were obtained.The results show that interband transition and bandgap narrowing have more obvious effects on α factor in semiconductor lasers(α factor are 22.562 and-6.853,respectively) than the effect of free carrier absorption(α factor is only-0.605).